As is well known, glaucoma produces a progressive loss (usually in a slow, characteristic fashion) of the retinal nerve fiber layer ("NFL") that is related to elevated intraocular pressure and other possible causative factors. The loss of NFL due to glaucoma leads to constriction of visual field, and may end in blindness. Up to now, there has been no reliable and sensitive method of diagnosing and monitoring glaucomatous damage to the NFL, or to visual function. Current clinical diagnosis of glaucoma relies on clinical history, intraocular pressure measurements, optic disc and NFL biomicroscopy and photography, and visual field testing. None of these methods alone has good sensitivity and specificity in the detection of glaucoma or glaucoma progression. However, taken together, they constitute a current diagnostic standard due to lack of better methods. Development of other techniques such as automated optic disc analysis by scanning laser ophthalmoscopy, and NFL assessment by scanning laser polarimetry has improved the reproducibility of measurements, but still has not demonstrated reliable and sensitive detection of glaucomatous damage.
Optical coherence tomography ("OCT") is a noninvasive technology that can produce cross-sectional images of tissues with ten (10) micron axial resolution. For example, see an article by D. Huang et al. entitled "Optical Coherence Tomography," Science, vol. 284, 1991, pp. 1178-1181. In particular, OCT can be used to image tissues within the human eye. For example, OCT systems suitable for non-contact imaging of vitreo-retino-choroidal structures are known in the art.
In OCT images, the NFL can be distinguished from other structures by its high reflected signal amplitude, see an article by Hee et al. entitled "Optical Coherence Tomography of the Human Retina," Archive of Ophthalmology, vol. 113, 1995, pp. 325-332 ("Hee et al."). Implementation of OCT for general tissue imaging, and for eye imaging, is disclosed in U.S. Pat. No. 5,321,501 to Swanson et al. In OCT imaging, a probe beam is focused on an eye structure of interest, and resulting optical reflections are analyzed in a low coherence interferometer. The interferometer resolves the signals reflected at various tissue depths by their optical delays. Lateral scanning of the probe beam provides information in a transverse dimension. The data can then be displayed as a tomograph, or cross-sectional image, of the tissue's reflected signal strength.
Computerized analysis of OCT images can yield quantitative information on the status of the NFL, and can have utility in the diagnosis and management of glaucoma and other optic neuropathies.
In investigating the retina, it is advantageous to scan an OCT beam in a predetermined path transversely across the retina, while at the same time, performing axial (depth) scans at multiple locations along the transverse scan path. For the purpose of measuring NFL, the scan path, as described in Hee et al., is a circle centered on the optic nerve head. By displaying all the axial scans together using a false-color or grayscale display, it is possible to create a two-dimensional "slice" of the tissue in which a vertical axis corresponds to depth in the tissue, and a horizontal axis corresponds to lateral position of the scan beam. A typical axial scan will show: (a) a high signal peak corresponding to inner retinal structures, i.e., the vitreo-retinal interface ("VRI") and the NFL layer; (b) a region of varied intermediate signals corresponding to the nuclear and plexiform layers; (c) a region of low signal corresponding to the photoreceptor layer ("PRL"); and (d) another region of high signal corresponding to the outer retinal and inner choroidal structures, i.e., the retina-pigment epithelium ("RPE") and the choriocapillaris.
One prior art method has been developed that detects the NFL based solely on relative reflectivity, see an article by Schuman et al. entitled "Reproducibility of Nerve Fiber Layer Thickness Measurements Using Optical Coherence Tomography," Ophthalmology, vol. 103, 1996, pp. 1889-1898 ("Schuman et al."). As described in Schuman et al., an image processing computer program determined total retinal thickness and retinal nerve fiber layer thickness for cylindrical OCT sections obtained by scanning around the optic nerve head (because axial information was obtained with OCT, circular scans produced a cylinder in three (3) dimensions). The images were corrected for artifacts due to involuntary subject motion during data acquisition using a standard image processing technique of cross-section scan registration. After subject motion in the longitudinal direction was corrected with a cross-correlation scan registration technique, a digital filter was applied to smooth the tomograms and reduce image speckle noise. Two-dimensional linear convolution with a center-weighted kernel was used to reduce speckle variations. Retinal thickness was quantitated by computer for each axial scan in the image as the distance between the first reflections at the vitreo-retinal interface and the anterior boundary of the reflective layer corresponding to the retinal pigment epithelium and choriocapillaris. Nerve fiber layer thickness was determined by computer. Boundaries were located by searching for the first points on each scan where the reflectivities exceeded a certain threshold. For example, an inner limiting membrane was located by starting anteriorly and searching downward in the image. A posterior margin of the nerve fiber layer was located by starting within the photoreceptor layer and searching upward in the image. The location of the photoreceptor layer was assumed to lie at the position of minimum reflectivity within the neurosensory retina. Thresholds were separately determined by the computer for each scan in the image as a fixed decibel level below the maximum signal in the scan.
Note that the above-described prior art method identifies NFL boundaries solely on the basis of a fixed relative signal strength regardless of where the maximum signal occurs, and does not take into account the fact that the signal from the outer retina is determined in part by how much light is reflected from the inner retina. Since NFL thickness is correlated with NFL signal strength, scans of tissue having minimal NFL will have too high a threshold, resulting in anomalously low thicknesses. In addition, attempting to locate an NFL boundary based on this threshold is subject to error, since signal fluctuation near the threshold value can lead to significant variation in the boundary.
As one can readily appreciate from the above, a need exists in the art for method and apparatus for diagnosing and monitoring eye disease such as, for example, glaucoma.